The present invention generally relates to direct-sequence spread spectrum (DSSS) communications systems, and particularly relates to characterizing multipath propagation characteristics in DSSS receivers.
In wireless communications systems, successfully extracting transmitted information from a received signal oftentimes requires overcoming significant levels of interference. Multipath interference represents one type of received signal interference that can be particularly problematic in certain types of wireless communications systems. For example, wireless LANs are typically employed in indoor environments that commonly include partitioned walls, furniture, and multiple doorways, along with various metallic and non-metallic building features. In these environments, transmitted signals follow multiple transmission paths of differing lengths and attenuation. Consequently, a receiver in such an environment receives multiple, time-offset signals of differing signal strengths. These multiple versions of the same transmit signal are termed xe2x80x9cmultipath signals.xe2x80x9d
The effect of multipath signals on DSSS receiver performance depends upon the particulars of the communications system in question. For example, in certain types of DSSS communications systems, multipath signals can actually improve receiver signal-to-noise ratio. To understand this phenomenon, it is helpful to highlight a few basic aspects of DSSS communications. DSSS transmitters essentially multiply an information signal by a pseudo-noise (PN) signalxe2x80x94a repeating, pseudo-random digital sequence. Initially, the information signal is spread with the PN signal, and the resultant spread signal is multiplied with the RF carrier, creating a wide bandwidth transmit signal. In the general case, a receiver de-spreads the received signal by multiplying (mixing) the incoming signal with the same PN-spread carrier signal. The receiver""s output signal has a maximum magnitude when the PN-spread signal exactly matches the incoming received signal. In DSSS systems, xe2x80x9cmatchingxe2x80x9d is evaluated based on correlating the incoming PN-sequenced signal with the receiver""s locally generated PN-sequenced signal.
The spreading code (PN code) used by the transmitter to spread the information signal significantly influences the effects of multipath signals on receiver performance. DSSS transmissions based on a single spreading code with good autocorrelation properties (or on a small set of orthogonal spreading codes) allow the receiver to selectively de-correlate individual signals within a multipath signal relatively free of interference from the other signals within the multipath signal. By adjusting the PN-sequence offset used to generate its local PN despreading signal, the receiver can time-align (code phase) its despreading circuitry with any one of the multipath signals it is receiving. If the spreading/despreading PN code has good autocorrelation and cross-correlation properties, the receiver can recover the transmitted data from any one of these multipath signals without undue interference. Of course, it may be preferable to use only the strongest multipath signal(s) for information recovery.
Indeed, conventional RAKE receivers used in Code-Division Multiple Access (CDMA) digital cellular telephone systems exploit the above situation. CDMA transmissions use a relatively long, fixed spreading code for a given receiver and transmitter pair, which results in very favorable auto- and cross-correlation characteristics. RAKE receivers are well known in the art of digital cellular receiver design. A RAKE receiver includes multiple, parallel xe2x80x9cRAKE fingers.xe2x80x9d Each RAKE finger can independently synchronize with and de-spread a received signal.
By synchronizing the multiple RAKE fingers to the strongest received multipath signals (those with the highest correlation values), the RAKE fingers lock on to the strongest multipath signals. Because of the excellent correlation properties of the CDMA spreading codes, each RAKE finger synchronizes with and de-spreads one of the multipath signals relatively free from interference associated with the other multipath signals. Thus, each RAKE finger de-spreads a relatively clean signal and this allows the overall RAKE receiver to coherently combine (with time/phase alignment) the signals to form a combined output signal that represents the addition of the multipath signals. Coherently combining the multipath signals allows the RAKE receiver to achieve an improvement in signal-to-noise ratio (SNR), essentially meaning that multipath signals can actually improve reception performance in certain types of spread spectrum systems.
Unfortunately, the characteristics of many other types of spread spectrum communications systems greatly complicate how a receiver deals with multipath signals. Some types of DSSS systems use spreading codes with poor correlation properties. The IEEE standard for high data-rate wireless LANs, known as 802.11b, is a primary example of such a system. Standard IEEE 802.11 transmissions use a single spreading code combined with binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK) to transmit data at 1 or 2 Mbps, respectively. The 802.11b extensions provide higher data rates by defining 5.5 and 11 Mbps transmission rates. These higher data rates use a modulation format known as Complimentary Code Keying (CCK). 802.11b CCK-mode transmissions use multiple spreading codes, and the spreading codes change across symbols. While providing the ability to achieve high data transfer rates and still maintain compatibility with the standard 802.11 and 2 Mbps channelization scheme, CCK modulation does include the drawback of making it more difficult for receivers to cleanly despread individual multipath signals.
Indeed, due to the relatively poor correlation properties of the spreading codes used in 802.11b, the various multipath signals can interfere with each other and result in inter-symbol interference (ISI) at the receiver. Thus, in contrast to the CDMA digital cellular scenario, multipath signals can significantly degrade receiver performance in systems operating under 802.11b standards. Of course, multipath signals may be problematic in any type of DSSS system that uses less-than-ideal spreading codes, so the problem is not limited to wireless LAN applications. Multipath interference in DSSS systems arises from both inter-chip interference (ICI) and ISI. For the purposes of this disclosure the term ISI is understood to include both ICI and ISI. From the perspective of a DSSS receiver, each transmitted symbol results in the reception of multiple symbols arriving with relative time offsets from each other, due to the multiple signal propagation paths between receiver and transmitter. ISI, as used herein, describes multipath interference arising from these multiple received symbols and can include interference arising from multipath signal delay spreads exceeding one symbol period.
In DSSS systems where the spreading code(s) do not allow multipath signals to be individually despread without interference, RAKE receiver techniques are not applicable. The basis of RAKE receiver operation assumes that each RAKE finger can cleanly despread a selected multipath signal, which is subsequently combined with the output from other RAKE fingers to form an overall RAKE receiver output signal. If the output from the individual RAKE fingers is corrupted by multipath interference, the combined signal will be compromised and RAKE receiver performance suffers.
Channel equalization offers a potential opportunity for improving receiver performance in a multipath channel. Unfortunately, conventional channel equalization techniques are not suitable for DSSS transmissions due to complexity. For any radio frequency channel, the term xe2x80x9cchannel-coherent bandwidthxe2x80x9d describes the portion of a given channel""s available bandwidth where a relatively flat frequency response may be observed. Typically, only a small portion of a wideband DSSS channel may exhibit a flat frequency response. Consequently, existing equalizers exploiting conventional filtering techniques are inappropriate for compensating a wideband DSSS channel for multipath interference. This inappropriateness results from the sheer complexity associated with implementing and training a conventional digital filter having a finite number of filter taps and corresponding filter coefficients that is capable of compensating the received signal for the complex frequency response of a wideband DSSS radio channel.
Existing approaches to DSSS receiver design do not adequately address multipath interference in systems where individual multipath signals cannot be despread relatively free of interference. As noted, these types of systems are typically based on less-than-ideal spreading codes, with IEEE 802.11b representing an example of such systems. Without the ability to handle multipath interference, such systems cannot reliably operate in environments with significant multipath interference. Existing approaches, including the use of RAKE receivers or conventional channel equalizers are either inappropriate or impractical.
Effective handling of multipath signals, whether for the purpose of interference compensation, such as in 802.11b environments, or for the purpose of coherent multipath signal combination, such as in RAKE receiver operations, depends upon developing accurate estimates of propagation path characteristics for one or more of the secondary propagation path signals included in the received signal. Under many real world conditions, the delay spread among the individual propagation path signals comprising a received multipath signal exceeds one symbol time, meaning that, at any one instant in time, the various propagation path signals may represent different information values (symbol values), making it potentially difficult to relate one propagation path signal to another. Without this ability, only multipath signals with propagation path delay spreads less than a symbol time may be processed substantially free from interference.
Thus, there remains a need for a method and supporting apparatus for identifying and characterizing secondary signal propagation paths relative to a main signal propagation path that accommodates a wide range of propagation path delay spreads, including delay spreads that exceed one symbol time. With the ability to determine time offsets between main and secondary signals over a range of less than to more than one symbol time, a communications receiver can accurately characterize secondary signal propagation paths relative to a main signal propagation path in variety of environments, even those with severe multipath conditions. Such a characterization method would allow for compensation of a received multipath signal in a broad range of radio signal propagation environments, even those with severe multipath conditions, thus enhancing communications receiver performance. This method and supporting apparatus would be particularly valuable in any type of DSSS communications system that relies on spreading techniques that do not intrinsically provide multipath interference rejection, but would also be valuable in any DSSS communications system subject to multipath signal reception.
The present invention provides a method of characterizing one or more secondary path signals relative to a main path signal, all such signals within a received multipath signal, thereby allowing a communications receiver or other like system to improve its reception performance in the presence of multipath signal reception. An exemplary embodiment of the method includes an initial course search that processes the received multipath signal to identify the strongest secondary signal or signals. With these signals identified, processing continues with comparing a magnitude of each of the identified secondary path signals with a magnitude of the main path signal to determine a more precise magnitude for the secondary path signals relative to the main path signal. This is followed by identifying corresponding symbols in said main and secondary path signals, which identifies any time shift in the secondary path signals relative to the main path signal. This is accomplished by identifying which symbols in a selected sequence of symbols received via the main path signal correspond which symbols in a sequence of symbols concurrently received via the secondary path signal. With time offsets relative to the main path signal identified for each secondary signal of interest, processing continues with determining phase shifts for each of the secondary signals relative to the main path signal. Thus, exemplary processing determines a phase shift, time offset, and magnitude for selected secondary signals relative to the main path signal. With this information, a communications receiver or like system can compensate the received multipath signal to improve reception performance.
In general, multipath signals comprise a strongest, main path signal, and remaining secondary path signals of varying lesser strengths (magnitudes). Each multipath signal has a potentially different magnitude, phase shift, and arrival time relative to the main path signal depending upon the characteristics of the particular signal propagation path through which it was received. Because each secondary path signal represents an altered version of the main path signal, secondary propagation path parameters may be determined based on comparing the secondary path signals relative to the main path signal. Thus, an information symbol or transmitted data item received through the main path signal may be considered to have an arrival time of t0, a phase of 0, and a magnitude of 1. Secondary path signals may then be compared to the main path signal to determine relative magnitude, phase, and arrival time, thereby characterizing the secondary signal propagation path parameters with respect to the main signal propagation path.
The method of the present invention operates on DSSS multipath signals in exemplary embodiments. The training method assumes that a communications receiver or other like system may be made to synchronize with the data timing (symbol timing) of the main path signal. Further, the training method assumes that the exemplary communications receiver supports an adjustable despreading operation, wherein the received multipath DSSS signal may be despread with a desired PN sequence phase offset with respect to the main path symbol timing. Initially, the training method steps the despreading PN code phase offset through a full phase cycle with respect to the main path symbol timing at a desired resolution (phase step size). This action is referred to as a xe2x80x9ccoursexe2x80x9d search and identifies main and secondary signals, and their associated PN sequence phase offsets (offset indexes) based on observing the correlation strength exhibited by the despread signal for each PN sequence phase offset.
Preferably, only the strongest secondary signals are selected for characterization, but the training method of the present invention allows any number of secondary signals to be characterized. After selecting the secondary signals of interest based on the results of the course search, each selected secondary signal is processed in combination with the main signal to determine the secondary propagation path parameters. This processing includes developing an accurate relative magnitude based on comparing secondary signal magnitude with main signal magnitude. Preferably, a sequence of transmitted symbols is received concurrently via the main signal and a selected secondary signal. By comparing the accumulated or integrated magnitude values for the secondary and main signal sequence, an accurate value of relative magnitude for the secondary signal may be determined.
Subsequently, a sequence of main and secondary signal symbol phase values are differentially decoded and accumulated (or integrated). Differential decoding removes any constant phase shift induced in the symbol values received through the secondary path relative to the main path. Preferably, the main signal phase values are converted to nominal values before differential decoding, thus eliminating noise from subsequent comparison operations. Also preferably, the secondary signal phase values are differentially decoded before being converted to nominal phase valuesxe2x80x94this helps to avoid erroneously choosing a nominal phase value before the unknown secondary signal phase shift is removed. These main and secondary signal differentially decoded phase samples are cross-correlated with each other to determine to what degree (time wise) the selected secondary signal either leads or lags the main signal. By incorporating techniques that accommodate an essentially arbitrary range of delay spreadxe2x80x94secondary signal arrival time with respect to the main signal arrival timexe2x80x94the training method accurately computes secondary path delays more or less than one symbol time away from the main path signal. These correlation operations identify the secondary signal path delay relative to the main signal.
Comparing leading, lagging, and current symbol phase samples, from both the main and secondary signal received symbol phase sequences determines a relative phase for the secondary signal. In fact, several possible relative phase values are determined using leading, lagging and current sample values. The previously determined secondary path time offset information determines the correct one of these possible phase values to use as the relative phase value. At this point, secondary propagation path parameters including relative magnitude, phase, and time offset are determined for a particular one of the selected secondary signals. The above processing may then be repeated for as many secondary signals as desired.
The relative magnitude and phase values may be combined to form a complex coefficient representing how the secondary signal is attenuated and phase shifted with respect to the main path signal. Thus, this complex coefficient may be applied to the main signal to produce an estimated secondary signal. These secondary signal estimates are useful in canceling one or more secondary signals from the received multipath signal. Indeed, in exemplary embodiments, the training method of the present invention provides a basis for multipath interference cancellation in wireless LAN systems based on IEEE 802.11b. In these applications, the training method of the present invention is used during the preamble and header portions of each 802.11b data packet to find the multipath signals, select the strongest (main) and next-strongest (secondary) multipath signals, and then characterize one or more of the secondary signals with respect to the main signal. This characterization information may then be used during the subsequent high data rate payload portion of the packet to provide cancellation of one or more of the secondary signals, thereby reducing interference in the main signal.
In other exemplary embodiments, the training method of the present invention may be used in DSSS communications systems to identify, select, and process main and secondary multipath signals for improvements in SNR, rather than for cancellation purposes. For example, a RAKE receiver may incorporate the training method of the present invention to identify and characterize the strongest multipath signals so that the individual RAKE fingers may be synchronized with a selected number of these strongest multipath signals. Then, the secondary path delay information developed in accordance with the training method supports the coherent combination operations performed by the RAKE receiver to gain improved SNR through multipath signal combining.